This invention relates to a system and process for production of methane gas by anaerobic digestion of feed materials containing suitable organic materials.
It is well known that suitable organic materials will decompose or decay by either aerobic (with oxygen) or anaerobic (without oxygen) processes. Both aerobic decomposition and anaerobic decomposition are naturally occurring biochemical processes. Since early times, man has exploited aerobic decomposition to produce compost for fertilizing his crops. Typically, compost production was accomplished with a compost pile. While some anaerobic decomposition always occurred in the compost pile, man did not begin to understand anaerobic decomposition and control it for useful purposes until the present century.
While either aerobic or anaerobic processes can be used for decomposing suitable organic materials, the resulting decomposition products will be quite different. If suitable organic materials are decomposed by an anaerobic process, both methane and carbon dioxide gases are produced. It is well known that methane is a colorless, odorless, tasteless, lighter-than-air gas which is useful as a fuel. In fact, methane is a principal component of ordinary natural gas. In addition to the production of methane, a potentially valuable sludge is produced when sewage, animal manures and many other feed materials containing suitable organic materials are digested by an anaerobic process. Such sludge can be used as a soil conditioner or as a source of various inorganic or organic elements and compounds useful as raw materials for subsequent manufacturing processes.
If anaerobic processes are to be successfully exploited for useful purposes, it is desirable to have those processes take place in insulated, oxygen-free containers. When feed materials containing suitable organic materials are placed in such containers, it is possible to both mimic and hasten the naturally occurring biochemical processes. When feed materials containing suitable organic materials are processed under such conditions, the process is commonly referred to as "anaerobic digestion". The insulated, oxygen-free containers are commonly referred to as "digesters".
Most of the early research and development relating to anaerobic digestion was performed in India. It is usually believed that the first digester for production of methane gas by anaerobic decomposition of organic materials was constructed in Bombay, India in 1900. That digester utilized cow manure as a feed material for the anaerobic decomposition process. It is well known that the Indian economy was at that time and continues to be in large part agriculturally based. It has been estimated that for every person living in India there are two cows living there. Because of this large cow population, large quantities of manure are available in India as a feed material for production of methane gas by anaerobic digestion. The resulting methane gas is burned as a fuel for cooking, heating, lighting and powering farm equipment. And, the resulting sludge is used as a soil conditioner.
In India, the research and development activities relating to anaerobic digestion have concentrated on simple small-scale digesters for digestion of manures and other organic waste materials. Both batch load digesters, i.e. digesters which are charged with feed material, sealed for the digestion period, and then emptied when methane gas production ceases, and continuous load digesters, i.e. digesters which are charged with smaller quantities of feed material on a daily or other periodic basis and continuously produce methane gas, have been developed, constructed and operated. The primary objectives for these digesters have been to provide digesters which are capable of being constructed by unskilled laborers using inexpensive building materials indigenous to a rural agricultural environment and to provide digesters which are capable of being operated and maintained by farmers and unskilled laborers having minimal formal technical education. Of course, these objectives are compatable with the circumstances existing in India.
In the United States and in other countries having highly industrialized economies, waste materials have not traditionally been considered to be a national asset. Accordingly, research and development activities relating to waste materials have concentrated on environmentally safe disposal of such materials. Nevertheless, some research and development activities relating to the production of methane gas by anaerobic digestion of feed materials containing suitable organic materials have been conducted in the United States. Until recently, most of these activities were conducted and financed by persons who are proponents of alternate life styles for the United States population. For this reason, the objectives of the early United States research and development activities relating to anaerobic digestion were similar to the objectives of similar activities in India. Small scale digesters were constructed for production of methane by digestion of waste materials containing suitable organic materials. In a few cases, larger digesters have been constructed. But, all of these digesters have been similar to those used in India since 1900 and have been used exclusively for digestion of waste materials.
With the continuing upward spiral of prices for oil, natural gas, coal and nuclear fuels, it is desirable to have a system and process for production of the maximum quantity of methane gas which a particular feed material containing a suitable organic material is capable of yielding. Such a system and process should be capable of using any material containing suitable organic materials as the feed material for the system and process. For example, it may be desirable to grow and harvest certain plants specifically for feed materials for the system and process.